A multitude of modular and composite thermally insulated door structures are known incorporating therein various compositions of materials primarily to provide a thermal barrier. When such doors have incorporated therein glass windows or thin panels, their structure is somewhat weakened and this will often cause the door to warp, and cause improper closing thereby causing air and water to infiltrate. U.S. Pat. No. 4,327,535 discloses the construction of a composite steel door incorporating a pane of glass which is surrounded by a mitered wooden frame and an interior foam core is disposed within opposed metal skins of the door. A top and bottom rail interlocks with the lock and hinge stiles which are fabricated of wooded boards. With this door structure the metal skins are secured to the stiles and rails, but there is no structural rigidity in between the stiles and rails, and the door is subject to warping along its longitudinal plane. A problem with using exposed indoor structures is that the steel will oxidate resulting in rust. Also, some door structures do not provide good insulation due to the fact that they utilize metal skins which are heat conductors and these skins contract and expand causing air and water leaks in areas where it is fastened or at joints.
It has now become customary to fabricate outside doors with plastic extruded material secured to wooden stiles and wherein a foam insulating material is injected within the door cavities. Such construction is, for example, illustrated in U.S. Pat. No. 5,074,087. A disadvantage of such door structures is that they exhibit weakness, particularly in the area where panels or glass panes are provided, and the door does not have rigidity in its longitudinal plane. Therefore, the door is apt to distort when the plastics material is subjected to heat and cold weather.
Many attempts have been made to construct reinforced domestic doors having a high resistance to warping, and this has led to complex door structures, with some such structures incorporating therein grids of reinforcing steel rods. These structures are very expensive and difficult to fabricate, and an example of such is disclosed in German Patent D2913003.